The present invention relates to a method for manufacturing photovoltaic devices, and more particularly, to a method for fabricating multi-cell photovoltaic devices.
A photovoltaic cell (also synonymously referred to as "photocells.") transforms optical energy into electrical energy and includes a junction between p-type and n-type semiconducting material which separates charge carriers generated by light. This junction is also referred to as a photocell impurity junction. Silicon is a common semiconductor material used in photovoltaic cells. Photovoltaic cells can be connected in series and/or parallel circuits to generate electrical power having various combinations of current and voltage. Typically, photocells are electrically connected in series to create multi-cell photovoltaic arrays.
A method for fabricating multi-cell photovoltaic arrays is described by Routh, et.al. in U.S. Pat. No. 4,156,309. The '309 patent describes the formation of a crystalline layer of silicon on an insulating substrate such as sapphire to fabricate a photocell array. If a photocell array is fabricated on a semiconducting substrate, the maximum voltage which can be obtained from the array is limited by the breakdown voltage of the junctions which isolate the individual photocells from the substrate. For an insulating substrate such as sapphire, the breakdown voltage is several orders of magnitude greater than that of a semiconductor. In accordance with the Routh method, the silicon layer is formed on the sapphire substrate by epitaxial growth, a process involving temperatures of about 950.degree. C., and the photovoltaic p-type to n-type impurity junction is formed by another high temperature process such as diffusion or ion-implantation. Diffusion requires temperatures of about 950.degree. C. and ion implantation requires an activation temperature of at least 850.degree. C. These high temperature processes have been found to cause fractures or defects in the silicon layer resulting from the differential in thermal expansion between the silicon layer and sapphire substrate. The temperature induced defects reduce the power conversion efficiency of the photovoltaic devices.
Therefore, there is a need for a process for manufacturing photocells and circuits employing photocells which does not produce thermally induced defects in the silicon comprising the photocells in order to maximize the energy conversion efficiency of these devices. Moreover, a need exists for a method of manufacturing a photovoltaic circuit capable of generating relatively high voltages, as for example, voltages exceeding 500 volts.